Optical path structure for open path emissions sensing with spinning filter wheel

ABSTRACT

An optical system for a gas component analysis includes an emitter for emitting first light beam having a first spectrum, a second emitter for emitting a second light beam at a second spectrum, a first receiver for receiving the first light beam, and a second receiver for receiving the second light beam.

PRIORITY

The present application is a continuation-in-part of pending U.S. patentapplication Ser. No. 09/934,272, filed Aug. 21, 2001, entitled OpticalPath Structure for Open Emissions Sensing, the disclosure of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to remote sensing systems. Moreparticularly, the present invention relates to an apparatus fortransmitting, reflecting, and detecting light in an open path sensingsystem such as a vehicle emission sensing system, having use indetecting and/or measuring one or more components of the air throughwhich the light passes.

BACKGROUND OF THE INVENTION

Current methods of determining whether a vehicle is compliant withemission standards include open path and closed path emissionsmeasurement systems. In a closed path system, an emission sensor isdirectly connected to the exhaust of the vehicle, such as by insertioninto a tailpipe. An open path vehicular emissions measurement systemcollects data by a means other than a direct connection to the tailpipe,such as a remote sensor that analyzes the individual components ofemissions. Open path vehicle emission systems are often preferable toclosed path systems because they can be used in numerous locations anddo not require the vehicle to stop for testing.

Various open path emission sensing systems have been known. One suchdevice uses a radiation source on one side of a roadway that projects abeam across the roadway to be received by a detector. The radiationsource and the detector are located on opposite sides of the roadway.The radiation source emits light spectra that may be used to detect anemission signature by way of absorption of light, or which alternativelymay be used to excite emission components so as to cause the componentsto emit light. The detected emission signature can then be used invarious applications, such as the measurement of a vehicle's compliancewith emission limits and the determination of the type of fuel that avehicle is using.

A disadvantage of many known arrangements is that the radiation sourcesand detectors must be placed on opposite sides of the roadway from eachother. Since both the detectors and radiation sources require power tooperate, this means that a separate power supply must be provided oneach side of the roadway.

Some known arrangements have tried to overcome this problem by using aradiation source on one side of a roadway and a reflective apparatuslocated on the other side of the roadway.

Accordingly, it is desirable to provide an improved opticaltransmission, reflection, and detection system as herein disclosed.

SUMMARY OF THE INVENTION

It is therefore a feature and advantage of the present invention toprovide an improved optical transmission, reflection and detectionsystem. In accordance with one embodiment of the present invention, agas component analysis system includes a first light source capable ofemitting a first beam of light having known emission intensitiescorresponding to one or more of infrared, visible, and ultravioletspectra. The system also includes a reflection unit, a detection unitcapable of receiving the beam and measuring received intensitiescorresponding to the plurality of light spectra, and a processor capableof comparing the received intensities and identifying a concentration ofa component corresponding to the intensities.

Preferably, the system also includes a first reflector positioned toreceive the beam from the first light source and reflect the beam towardthe reflection unit. The reflection unit is positioned to receive thebeam from the reflector and reflect the beam. Also preferably, a secondreflector is positioned to direct the beam reflected by the refectionunit so that the beam may be received by the detection unit. Eachreflector preferably comprises an off-axis paraboloidal mirror.

Also preferably, the system also includes a filter wheel positioned tospin about an axis and receive the beam from the first light source andpass the beam to the reflection unit in pulses. The filter wheelpreferably includes a plurality of filters, each of which substantiallylimits the passage of light to a predetermined spectral wavelength orrange of wavelengths.

Also preferably, the first beam of light travels along an optical pathto the reflection unit. In this embodiment, the system also includes asecond light source capable of emitting a second beam of light havingknown emission intensities corresponding to one or more of infrared,visible, and ultraviolet spectra, as well as a beam splitter/combinerpositioned to direct the second beam of light along substantially thesame optical path to the reflection unit.

In an alternate embodiment, the system also includes a spinningreflector positioned to spin about an axis and receive the beam from thereflection unit and direct infrared components of the beam to thedetection unit in pulses.

In accordance with another embodiment, a method of measuringconcentrations of one or more components of a gas includes the steps of:(1) emitting at least one beam of light having known emissionintensities corresponding to a plurality of infrared, visible, andultraviolet spectra through the gas; (2) using a reflection unit toreflect the beam; (3) using a detection unit to receive the beam; (4)measuring received intensities in the beam corresponding to theplurality of light spectra; and (5) identifying a concentration of atleast one component of the gas corresponding to a ratio of the emissionintensities and the received intensities.

Preferably, the method embodiment also includes, either before or afterthe reflecting step, filtering the beam and passing the beam to thereflection unit in pulses. It may also include, before the detectingstep, directing infrared, visible and ultraviolet components of the beamto different detectors and/or spectrometers in the detection unit. Alsopreferably, in the method embodiment identifying step is performed by aprocessing device that is programmed to perform the calculation of acomponent concentration using a formula corresponding to theBeer-Lambert law.

In another embodiment, the invention provides, an optical system for agas component analysis. The system has a first emitter for emitting afirst light beam having a first spectrum; a second emitter for emittinga second light beam at a second spectrum; a first receiver for receivingthe first light beam; and a second receiver for receiving the secondlight beam. The first light beam travels along a first path in a firstdirection and the second light beam travels along a second path in asecond direction and at least a portion of the first light path overlapswith at least a portion of the second light path to firm an overlappingbeam, and at the overlapping beam the first direction is opposite to thesecond direction.

In another aspect, the invention provides an optical system for a gascomponent analysis, that has a first emitter located on a first side ofa vehicle path for emitting a first light beam having a first spectrumacross the vehicle path, a first receiver for receiving the first lightbeam, and a spinning filter wheel that filters the beam from the firstemitter before the beam crosses the vehicle path.

In another aspect, the invention provides an optical system for a gascomponent analysis has a first emitter located on a first side of avehicle path for emitting a first light beam having a first spectrumacross the vehicle path, a first receiver for receiving the first lightbeam, a plurality of filter elements, and a spinning mirror face thatreflects the beam so that the beam reaches each of the filter elementsin sequence.

There have thus been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thereofthat follows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described below andwhich will form the subject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments and of being practiced and carried out in various ways.Also, it is to be understood that the phraseology and terminologyemployed herein, as well as the abstract, are for the purpose ofdescription and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a preferred embodiment of a source unit of thepresent invention including housing with window, light sources, filterwheel, beam splitter/combiner, and reflector.

FIG. 2 illustrates a preferred embodiment of a reflection unit of thepresent invention.

FIG. 3 illustrates a preferred embodiment of a detection unit of thepresent invention including housing with window, reflector, beamsplitter/combiner, detector and spectrometers.

FIG. 4 illustrates an exemplary filter wheel that may be used inaccordance with one embodiment of the present invention.

FIG. 5 illustrates an alternate embodiment of a detection unit of thepresent invention including housing with window, reflector, beamsplitter/combiners, spectrometers, spinning reflector, monolithicellipsoidal mirror, filter array with gas cells, focusing reflector, anda single infrared detector.

FIG. 6 illustrates several elements of an exemplary computer of a typesuitable for carrying out certain functions of the present invention.

FIG. 7 illustrates a detection unit using multiple spectrometers and asingle detector.

FIG. 8 illustrates the properties of an ellipsoidal reflector.

FIG. 9 is a conceptual diagram of some basic components of the presentinvention, including light source, reflection unit, detection unit, andprocessor.

FIG. 10 illustrates the addition of reflectors to the components of FIG.9.

FIG. 11 illustrates the properties of a paraboloidal reflector.

FIG. 12 further illustrates the properties of a paraboloidal reflector.

FIG. 13 illustrates the addition of multiple light sources with beamsplitter/combiners to the components of FIG. 10.

FIG. 14 illustrates a modification of the embodiment shown in FIG. 13illustrating the arrangement of opposed sources.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A preferred embodiment of the present invention provides an improvedoptical source, reflection, and detection system for gas componentanalysis. A preferred embodiment includes a light source unit, whichpreferably includes one or more of infrared, visible, and ultravioletlight sources; a reflection unit; and a light detection unit.Preferably, light sources and detectors are contained within a housing.The light is transmitted through a gas, such as air containing vehicleemissions, reflected, and detected for analysis and measurement of theamount of absorption that has occurred at known wavelengths of thelight. The amount of absorption may be used to determine concentrationsof gases corresponding to the specific wavelengths.

In a preferred embodiment of this invention, infrared, visible, andultraviolet radiation is combined into one beam, directed across a pathsuch as a road along which vehicles travel and generate exhaust,reflected back across the path, collected and concentrated, separatedagain, and received by one or more discrete detectors and/orspectrometers. In order to be able to separately analyze each range ofwavelengths, the infrared light passes through a sequence of filtersand/or gas cells either before or after traversing the path of lightacross the road. The filters are preferably narrow band pass filters andthe gas cells contain known concentrations of gases of interest, suchthat each filter or combination of filters and gas cells is specific toa gas of interest. In one embodiment, a spinning wheel holds the filtersand passes each filter in front of the infrared light source insequence, before the light traverses the road. In an alternateembodiment, the infrared light, after traversing the road, isdistributed by a spinning reflector, such as a mirror, into a stationaryarray of filters and/or gas cells in sequence to an ellipsoidal mirroror an array of ellipsoidal mirrors that focus the light into a singledetector. The visible and ultraviolet light is directed to one or morespectrometers that can analyze the desired wavelength ranges directly.

A portion of a preferred embodiment of the present inventive apparatusis illustrated in FIG. 1. FIG. 1 illustrates a possible light sourcecomponent of the present invention. The light source component shownincludes an infrared light source 10, a source of visible light 11, andan ultraviolet light source 12. The infrared light 14 emitted by theinfrared source 10 passes through a filter wheel 16, more completelydescribed in FIG. 4. Then it is reflected by a beam splitter/combiner18, and follows an optical path 20 until it reaches a reflector 26 suchas an off-axis paraboloidal mirror or spherical mirror. An off-axisparaboloidal mirror is preferred over a spherical mirror due to theaberrations in light that occur with spherical mirrors, howeverproduction economics may dictate the use of spherical mirrors. Thereflector 26 reflects the infrared light along a path 22, through aprotective window 25 in the housing 27, leading to a reflection unitillustrated in FIG. 2.

The reflector 26 and other optical components described in thisembodiment are protected by a window 25 that allows the transmission ofall of the wavelengths of interest. This window 25 is attached to thehousing 27 of the entire source unit. Preferably, the light sources anddetectors are included within a single housing. However, the lightsources and the light detectors may optionally be provided in more thanone housing. Also preferably, the housings are sealed to preventcontaminants such as soot, road dust, and other road debris fromdamaging or coating the internal components and thus degrading the lightsignal received and/or transmitted by them. Also preferably, the sealedhousings contain windows to allow light of the wavelengths of interestto leave and enter the housings as required for the light to travelalong the desired optical path. These windows are preferably made of amaterial such as calcium fluoride (CaF₂), sapphire, or other materialthat will pass light of all wavelengths of interest with little or noattenuation. Optionally, the windows may be coated by a particular typeof coating such as an anti-reflection coating or other suitable coatingto enhance the transmission of light of the wavelengths of interest.

The infrared light source 10 may be any source that emits a sufficientintensity of light of the wavelengths of interest. The reflectors andoptical path length determine the size of the spot from the infraredsource that contributes to the light beam. Preferably the source ischosen, such that the light emitting area of the filament is as close tothat spot size as possible for minimum power consumption.

Preferably, the filter wheel 16 is a spinning wheel that is powered by amotor 15 that spins the wheel 16 about an axis 19. Also preferably, asynchronization device 58 is provided to track the position androtational speed of the filter wheel 16. Features of the filter wheel 16are more completely illustrated in FIG. 4.

In addition, visible light from source 11 is focused by an opticalelement 13 to bring diverging light rays back into a focus through thecenter of ultraviolet source 12 where it is combined with theultraviolet light from source 12 into a combined beam 24. The combinedvisible and ultraviolet light 24 passes through the beamsplitter/combiner 18 such that it also follows optical path 20 to thereflector 26, where the light is reflected to also follow path 22 outwindow 25 toward the reflection unit illustrated in FIG. 2. The visiblelight source 11 may be a light emitting diode (LED), which emits lightin a narrow range of wavelengths, or another visible source such as ahalogen lamp that emits a broader range of wavelengths. The advantage ofpassing the visible light through the ultraviolet light source iseliminating the need for another beam splitter/combiner, saving opticalpower that would otherwise be lost by the inefficiency of the beamsplitter/combiner, in addition to saving space within the enclosure 27.However, if it is desirable to have an ultraviolet source 12 of a designthat does not allow for pass-though of the visible light, thenalternatively, the visible source 11, and ultraviolet source 12 may bereconfigured to take, for example, positions 146 and 144 as illustratedin an arrangement of sources in FIG. 13 that will be discussed furtherbelow.

The visible light source 11 is an option and is not required for gaseousmeasurements. However, such a source allows the performance ofadditional tests on detected light associated with visible spectra, suchas an opacity test. One type of opacity test that may be performed isthe Society of Automotive Engineers J1667 opacity test, also known asthe “Snap Acceleration Test”, which measures concentrations of lightspectra in the range of 562 through 568 nanometers. Preferably, thevisible source 11 can be sufficiently broad in its output wavelengthcharacteristics to also permit detection of blue smoke, which may beindicative of a vehicle that is burning excessive lubricating oil, andparticulate matter of varying sizes. Alternatively, more than onevisible source optimized for specific causes of exhaust opacity may beused.

The ultraviolet light source 12 is preferably an ultraviolet lamp suchas deuterium lamp, a xenon lamp, or another lamp that has ultravioletlight emission characteristics broad enough to include wavelengths ofinterest.

As FIG. 1 illustrates, where multiple light sources such as components10, 11, and 12 are provided, the emitted beams preferably followsubstantially the same optical path 20 toward the reflector 26. Thereflector 26 is positioned such that light sources 10, 11, and 12 arenear the focal point of the reflector 26 and the reflected light 22 isparallel to its axis of rotation. The angle between the incoming 20 andreflected light 22 and the focal length are determined by the design ofthe reflector 26 and may be chosen based on considerations of componentlayout and F-number. (F-number of an off-axis paraboloidal mirror isdefined as the diameter of the mirror divided by its effective focallength.) Thus, light 20 transmitted to the reflector 26 is reflected ina direction 22 that is away from the original light sources 10, 11, and12. In addition, if beam splitter/combiner 18 is a neutral densityfilter, it is preferably chosen so that the proportion of visible andultraviolet light passed and the proportion of infrared light reflectedare balanced according to the requirements of the detection unit.Optionally, a beam splitter/combiner 18 that is sensitive to differentwavelengths such as a dichroic beam splitter may be used instead of aneutral density filter for beam splitter/combiner 18. In order to usesome types of beam splitter/combiners, the positions of the infrared 10and visible/ultraviolet sources 11, 12 may be reversed.

Optionally, only the infrared light source 10, and not the visible orultraviolet sources 11, 12, may be provided. In such an embodiment, thebeam splitter/combiner 18 may also optionally be omitted, with theinfrared light source 10, filter wheel 16, and associated components,taking the position of the ultraviolet source 12 in such an alternativearrangement. This option of the preferred embodiment may be desired formore economical utilizations of this embodiment where not all exhaustemissions constituents are desired to be measured, or a preferenceexists to simplify the production of such an embodiment with thepotential compromise of poorer data quality.

FIG. 2 illustrates an exemplary reflection unit, which in an embodimentused to detect vehicle emissions is preferably placed across the roadfrom the light source and detector components, creating an open-pathemissions testing system. The reflection unit includes aretro-reflective system, preferably a vertical system, and preferablycomprising three mirrors positioned to form 90° angles with respect toeach other. A vertical orientation of the mirror assembly is preferredin order to adequately capture the emissions of vehicles of all profilesand heights. Referring to FIG. 2, incoming light 22 is reflected by afirst mirror 30 and a second mirror 32. The first and second mirrors areadjacent or substantially adjacent to each other to form a 90° angle.The light reflected by the first and second mirrors is transmitted to athird mirror 34. As FIG. 2 illustrates, the flat reflective portion ofthird mirror 34 forms a 90° angle with the flat reflective portions ofboth first mirror 30 and second mirror 32. It is not important to havemirrors 30,32 on top of mirror 34, as this orientation could be reversedwithout any change to the quality of reflection of light. Light 36 thatis reflected by third mirror 34 is then transmitted to the detectionunit and travels in a direction that is parallel to the incoming light22 in a configuration as illustrated in FIG. 9 to be discussed later inthis text. The incoming light 22 and/or the reflected light 36 passthrough an air component that is to be measured, such as vehicleemissions.

FIG. 3 illustrates an exemplary detection unit that receives the lightthat is generated by the source component of FIG. 1, and reflected bythe reflection unit of FIG. 2. Referring to FIG. 3, incoming light 36passes through a protective window 35 that has similar characteristicsto the window of the source unit illustrated in FIG. 1, is reflected bya reflector 38 such as an off-axis paraboloidal mirror or sphericalmirror that reflects light along an optical path 40 at an angle relativeto the incoming light 36. The light transmitted along the optical path40 is reflected by a beam splitter/combiner 44 that directs infraredlight 48 toward infrared detector 50. Preferably, the infrared detector50 is positioned within the focal volume so that the light willover-bathe the detector's active area to allow for system vibrationwithout adversely affecting measurements by causing a portion of thedetector's active surface to temporarily not have light exposure in avibration occurrence. Focal volume is defined as the three-dimensionalvolume of light, in which the light is focused to its maximum intensity,in this instance infrared light 48, that travels to the detector 50.Maximum intensity of light occurs when all lights rays are concentratedinto the smallest cross-sectional area of the focal volume. Thiscross-sectional area is not necessarily located at the focal point ofthe reflector 38, but is located farther away from the reflector 38 thanthe focal point.

Small, economical, durable, and versatile spectrometers 42, 43 arecommercially available for most ranges of wavelengths of interest in thevisible and ultraviolet regions. In the infrared region, however,spectrometers are less practical than individual detectors optimized forparticular ranges of wavelengths. These infrared detectors are expensiveand require cooling and complicated electronics for support. It istherefore a great advantage to use only a single infrared detector 50 inthe detection unit. If separate detectors are used to detect theintensity of each wavelength or band of wavelengths of interest, thecalibration problem caused by the different sensitivities of thedifferent detectors must be addressed. This problem is furthercompounded because sensitivities change with time and temperature andcan be different for each detector. Therefore a system using only asingle infrared detector 50 is much simpler and is preferred.

The infrared detector 50 is preferably composed ofmercury-cadmium-telluride (MCT), preferably utilizing at leastthree-stage thermal electric cooling. However, a lead-selenide or othercomposition detector can be used, and with greater or lesser stagedcooling. A liquid cooled detector could also be utilized in thisembodiment provided there is supporting equipment to accommodate theliquid cooling. Another possibility for cooling the detector is byStirling Engine cooling, however this adds cost and complexity. The MCTcomposition detectors offer a more compatible electronic biasingconsistent with reduced noise than other composition detectors. Otherfactors considered for single detector selection is the detectivity,commonly expressed in terms of “D*”, responsivity to light, the timingof the pulses of light to which the detector is exposed, and thesaturation level.

This embodiment also prefers the economy of a photoconductive type ofsingle detector as opposed to the more expensive photovoltaic detector.While photovoltaic detectors comparably offer less noise in lower pulsefrequencies, this is not an issue for this embodiment as it is desirableto stimulate the detector with as high a frequency that the spinningfilter wheel illustrated in FIG. 1 item 16, or spinning reflectorillustrated in FIG. 5 item 62 will allow.

Lastly, a detector needs to be selected to respond to light consistentwith the range of desired wavelengths. A range of mid-infraredwavelengths for this embodiment can be viewed in Table 1 which suggestsa detector sensitivity range of wavelengths between roughly 3-5 microns.However, if alternative wavelengths are used for such embodiment tomeasure the gases of interest, the desired range of wavelengths to whichthe detector is sensitive may have to be adjusted.

If the range of infrared wavelengths of interest is too broad for astandard detector, a dual substrate detector may be used. A commerciallyavailable dual substrate detector contains two different semiconductorcompounds, each sensitive to slightly different ranges of wavelengths.They are mounted in a single detector package, one in front of the otherso that their active areas nearly coincide. Thus the combinationperforms as if it were a single detector with sensitivity to a broaderrange of wavelengths than would otherwise be possible.

The beam splitter/combiner 44 may comprise any reflective ortransmissive device, such as a neutral density filter, which transmit aspecified fraction of the incident light and reflect almost all of therest, treating a broad range of wavelengths equally, or dichroic beamsplitter/combiner that can be designed to reflect almost all of theincident light of a specific range of wavelengths, and transmit almostall of the rest. The beam splitter/combiner 44 passes all or portions ofvisible and/or ultraviolet light 46 so that the visible and ultravioletspectra may be measured by one or more spectrometers 42, 43. The lightwhich passes through beam splitter/combiner 44 is split off and carriedto the respective spectrometers in one of two ways. The first,illustrated in FIG. 3, is to focus light onto the end of a Y-shapedoptical fiber cable 41 that first receives the light in a single openend of the fiber optic cable, then divides the light within the cablesending a portion of the light to each spectrometer.

An alternative method of splitting the light to two or morespectrometers, illustrated in FIG. 7, is to use separate beamsplitter/combiners 44 and 162 to split light beam 40 twice. Beamsplitter/combiner 44 first splits beam 40 into beams 170 and 172. Beam170 is focused directly into the opening of spectrometer 43 while beam172 continues on to beam splitter/combiner 162. Beam splitter/combiner162 then splits beam 172 into beams 174 and 176. Beam 174 is focused onspectrometer 42 while beam 176 continues on to be focused on theinfrared detector 50. In either embodiment, whether cable splitting oflight as illustrated in FIG. 3 or multi-beam splitting method of FIG.11, the light slightly over-bathes the opening to the optical fibercable (FIG. 3 item 41) or the light orifice of the spectrometer 42, 43for resistance to vibration and coincident reduction of light intensitywith the vibration for similar reasons as expressed above for theinfrared detector 50.

TABLE 1 List of Some Example Tailpipe Emissions Channels and theirWavelengths Component Wavelength Carbon Monoxide (CO) 4.65 μ CarbonDioxide (CO₂) 4.30 μ HC₁ (Alkane series hydrocarbons) 3.45 μ Methane(CH₄) 3.31 μ HC₂ (Alkene series hydrocarbons) 3.17 μ HC₃ (Alkyne serieshydrocarbons) 3.01 μ H₂O_((v)) 2.90 μ Nitrogen Monoxide (NO) 0.226 μ 1,3Butadiene (C₄H₆) 0.210 μ Ammonia (NH₃) 0.208 μ Reference 3.90 μ

In a preferred embodiment, the transmission and detection of light atthe wavelengths of mid-infrared listed in Table 1 is accomplished byusing a spinning filter wheel as the filter component (referred to inFIG. 1 as item 16). FIG. 4 illustrates an exemplary spinning filterwheel. Referring to FIG. 4, the spinning filter wheel contains lightfilters such as 52 that correspond to wavelengths associated withindividual emission components, such as those illustrated in Table 1.One of the filters 54 must correspond with a wavelength at which nogaseous absorption takes place. Such a filter is known as a “reference”filter 54. The light intensity measured from the reference filter 54 isused to normalize the light intensity measured from each of the gaseousfilters 52, so that concentrations of those gases may be calculated by aprocessor (FIG. 6 item 92). FIG. 4 illustrates a wheel having eightfilters 52, 54 each utilizing one of the mid-infrared wavelengths ofTable 1, however fewer and/or additional filters, corresponding to fewerand/or additional vehicular exhaust constituents, may be used inalternate embodiments. Each filter 52 is designed to allow light of aspecific range of wavelengths to pass through it.

Another innovation regarding the filters 52, 54 is that they arequadrants of an industry standard 25 millimeter optical filter. Theround, 25 millimeter diameter filters are cut into four pie shapesallowing for filters to cost one-fourth of what they would otherwisecost if an entire industry standard sized filter were to be inserted ineach of the open positions on the filter wheel 16. In addition to cost,there is a savings in the amount of rotating mass by quartering theindustry standard sized filters that the wheel 16 would have if thefilters were installed whole. Lastly, special slots exist in the wheel16 to allow for a two-piece optical filter 52,54, should this benecessary. There are occasions when a filter manufacturer will supplytwo filters in order to provide the desired band pass of wavelengths tomeasure a gas of interest. The wheel 16 has the capability to acceptthese two-piece filters.

In addition, the filter wheel preferably will have one or moresynchronization marks 56 that may be detected by a synchronization unit58 to define either the exact filter or the start of a sequence offilters that will be in the optical path. The wheel 16 must have anopaque area 60 between each filter. The opaque areas 60 prohibit sourcelight (FIG. 1 item 10) from getting to a detector when the opaque areas60 pass in front of the infrared source (FIG. 1 item 10) transformingthe incident light beam into a sequence of pulses (FIG. 1 item 17). Inoperation, the wheel spins about an axis 19 at high speeds, preferablyat least 12,000 rotations per minute, to form a sequence of infraredlight pulses (FIG. 1 item 17). Faster rotational speeds are even morepreferable since they increase the sampling rate of the emission medium.The synchronization unit (FIG. 1 item 65) allows the processor (FIG. 6item 92) to associate a wavelength of interest, and corresponding gas ofinterest, with each pulse of light seen by the detector (FIG. 6 item90). This combination overcomes disadvantages of prior art, whichrequire discrete detectors for each wavelength.

In accordance with an alternate emodiment of the present invention thelight source unit illustrated in FIG. 1 may omit the spinning filterwheel assembly 15, 16, 19, 58. In this embodiment, an alternate detectorunit is provided as illustrated in FIG. 5. Incoming light 36 transmittedfrom the source unit of FIG. 1 and reflected by the reflection unit ofFIG. 2 passes through window 35 that has similar characteristics towindow of source unit illustrated in FIG. 2 passes and is reflected by areflector 28, which directs the light beam 40 onto beamsplitter/combiners 44, 45 which direct portions 46, 47 of the light tothe spectrometers 43, 42. Beam 49 passes from splitter/combiner 44 to45. The rest of the light 61 is focused on spinning reflector 62.Reflector 62 is a single faceted flat mirror with a reflective surfacethat is optimized for the infrared light wavelengths of interest, suchas an enhanced gold reflective surface or other suitable reflectivesurface. Alternatiely, a multifaceted spinning mirror may be used,however the geometry of the rest of the layout would have to be modifiedfrom what is illustrated in FIG. 5. The spinning reflector 62 splays thelight in sequence around a stationary array of filters 52, 53, 54 andgas cells 70 by directing the beam 64 into the side of monolithicellipsoidal mirror 80 which reflects the light 66 into the array,consistent with the splaying of the light. After passing through eachstationary band pass filter 52, 53, 54 and gas cell 70, the light beam72 is redirected to and focused on single infrared detector 50 by areflector 74 such as a sperical mirror. The reflective surfaces ofreflectors 80 and 74 are optimized for the wavelengths of interest inthe same way as the surface of spinning reflector 62. The singleinfrared detector sees a sequence of pulses of light 76 that areessentially the same as those illustrated as FIG. 3 item 48. Each filter52, 53, 54 of this array substantially limits the passage of light to apredetermined spectral wavelength or range of wavelengths. Some filtercenter wave specifications are listed in Table 1. Each gas cell 70 ofthis array substantially limits the passage of light of a particularspectral pattern of wavelengths absorbed by the known concentration ofthe gas of interest that the cell 70 contains.

Another advantage of this embodiment is that there is much less rotatingmass in the spinning reflector 62 than in the spinning filter wheelillustrated in FIG. 4. Therefore the spinning reflector 62 can be spunat a much faster rate than the spinning filter wheel illustrated in FIG.4. Faster spin rate corresponds to a higher sampling rate that cancontribute to lower electronic and optical noise levels, and providebetter time resolution of a plume of vehicle exhaust constituents.

It is instructive to refer to the illustration of FIG. 8 to further theunderstanding on why an ellipsoidal mirror (FIG. 5 item 80) is chosen todistribute light. An ellipsoidal mirror 200 has two focal points or foci206, 208. Such mirrors have the property that all light rays 202, from asource 204, diverging from a small spot near one focal point 206 arereflected in such a way that those rays 210 are again focused into asmall spot near the other focal point 208 of the mirror 200. Given theunique layout of the alternative embodiment of FIG. 5, and commensurateneed for a dual foci reflective device for light distribution through afull 360° of rotation of the spinning reflector (FIG. 5 item 62), anellipsoidal mirror is the bet choice for this alternative embodiment.

An alternative embodiment replaces the monolithic ellipsoidal mirror 80with individual ellipsoidal mirrors and may place the filters 52, 53, 54and gas cell 70 array before the individual ellipsoidal mirrors iflayout and construction is simplified. This alternative can provide theadvantage of the system suffering less light loss through use ofindividual mirrors as opposed to the monolithic ellipsoidal mirror 80.The disadvantage is that there may be more adjustments required in orderto have the system of FIG. 5 properly aligned such that all lightthrough the system is optimized.

FIG. 6 illustrates several elements of a computer processing device thatmay be used in accordance with a preferred embodiment of the presentinvention. Referring to FIG. 6, the detection unit 90 deliversemissions-related data to a processor 92. The detector may be any of thedetectors or spectrometers as illustrated in FIGS. 3 and 5, or anydevice that receives or contains information collected by such detectorsor spectrometers. Such detector systems for the purpose of discussion inFIG. 6 include a means for amplifying and converting the detectorsignals into digital signals that can pass to the processor 92 via adirect link such as a parallel data bus 94.

In this embodiment, the detection unit 90 is part of the unit thatcontains the processor 92, and the delivery is performed over a parallelbus 94 such as that which can be found in AT, ATX, EBX, and othermotherboard styles upon which computers are based. However, theprocessor 92 and detection unit 90 may be separate, such as with theremote detector 96 illustrated in FIG. 6. Where a remote detector isused, the data may be delivered to the processor 92 by a communicationslink 100 that delivers the data to an input port 98 such as acommunications port. A wireless communications link 102 and receiver 105for such a wireless communication are also illustrated in FIG. 6. Thecommunications link 102 may be a direct wire, an infrared data port(IrDA), a wireless communications link, global communications networksuch as the internet, or any other communications medium.

The system illustrated in FIG. 6 also includes a memory 104 which may bea memory device such as a hard drive, random access memory, or read onlymemory. A portion of this memory 104 can contain the instructions forthe processor 92 to carry out the tasks associated with the measurementof vehicular emissions. Preferably, concentrations of gases may bederived using the Beer-Lambert Law, however other tests and formulae maybe used in alternate embodiments.

The Beer-Lambert Law, as disclosed in other art, relates absorbance oflight to a concentration of gas where an amount of change in lightintensity at a known wavelength is proportional to the concentration ofa gas of interest at the wavelength of light where the gas is absorbed.The Beer-Lambert Law is expressed in terms of transmittance in Equation1.

Equation 1: Beer-Lambert Law

2−Log₁₀(% T)=εCl

Where:

% T is the amount of light transmitted through open air and theemissions sample expressed in percent units;

ε is the absorption coefficient for the gas of interest at acorresponding wavelength of absorption;

C is the concentration of the gas of interest expressed inparts-per-million (ppm)

l is the path length expressed in meters.

Transmittance is further expressed as the amount of light that passesthrough the gas of interest in proportion to the amount of light thatwas originally emanated from the light source unit as illustrated inEquation 2.

Equation 2: Transmittance as Expressed in Percent

${\% \quad T} = {\frac{I_{p}}{I_{o}} \times 100}$

Where:

I_(p) is amount of light left after passing through the gas sample ofinterest

I_(o) is the amount of light that was originally sent through the entiresample path and not absorbed by the gas of interest.

The specific application of Beer-Lambert Law for this embodiment isfound in Equation 3. Equation 3 is an algebraic substitution oftransmittance “% T” (Equation 2), and subsequent manipulation ofBeer-Lambert Law Equation 1 to solve for a concentration of a gas in anopen path, as this is the unknown for which this embodiment measures.

Equation 3: Application of Beer-Lambert in this Embodiment

$C = \frac{2 - {{Log}_{10}\left( {\frac{I_{p}}{I_{o}} \times 100} \right)}}{ɛ \times l}$

Other memory devices 106 and 108 such as additional hard disk storage, aCD-ROM, CD-RW, DVD, floppy drive, ZIP® drive, compact flash compatibledevice such as that which conforms to IBM Microdrive™ specification, orother memory device may also be included. An internal memory device 106can be used to extend the number of emissions tests that can beconducted and retained by this preferred embodiment. A removable memorydevice 108 can be used to make the emissions data portable to allow forthe emissions data to be further processed in a centralized location.The device also optionally and preferably includes a display 110 and/ora transmitter 112 for providing output to a user or another device.

Utilizing a computer processor 92, the intensity measured by thedetector unit 90 at a wavelength of interest is compared by theprocessor 92 to the intensity of light detected by the detector unit 90at a reference wavelength where no absorption of gases occurs. Thismethod of detection is commonly known as Differential Optical AbsorptionSpectroscopy (DOAS). This DOAS methodology is a simple, inexpensivemeans of determining a concentration of a gas of interest emanating froma vehicle tailpipe in open air, and has examples in other art and fieldsof invention.

Alternatively, again using a computer processor 92, the intensitymeasured by a detector unit 90 at a desired wavelength for an intervalof time, followed by measuring light at the detector unit 90 for aninterval of time at the same desired wavelength with additionally a gascell of known concentration of gas that absorbs light of the samewavelength can also be used as a methodology to determine aconcentration of a gas of interest. This method of detection is commonlyknown as Gas Filter Correlation Radiometry (GFCr), and is documented inother art. GFCr has the potential to provide improved precision &accuracy of measurements due to the fact that the methodology allows forthe constant referencing of a measurement to a known concentration ofthe gas of interest.

A preferred embodiment of FIG. 5 shows both DOAS and GFCr methods ofdetermining a concentration of a gas of interest contained within thesame embodiment. For example, an optical filter 53 can be optimized forsampling carbon dioxide (CO₂). Another filter 54 can be optimized topass wavelengths of light where no absorption of CO₂ or other gasesexist; such a filter is used for reference to assess the amount of lightthat passes through the sample path without CO₂ influence. As the amountof CO₂ concentration increases, the amount of light that the detector 50observes through filter 53 will decrease, while the amount of light thatthe detector 50 observes through the reference filter 54 will remainunchanged. This is the fundamental of the DOAS methodology by comparingthe amount of light (I_(p) in Equations 2 and 3) off from the CO₂ filter53 to the amount of light (I_(o) in Equations 2 and 3) from thereference filter 54. Switching the light paths between the CO₂ path,created by filter 53 to detector 50, and reference path, created byreference filter 54 to detector 50, is accomplished by the spinningreflector 62 that splays the light for periods of time between the twomentioned paths and other paths that exist in this embodiment.

DOAS methodology is also provided in the embodiment illustrated in FIG.1, however the light path switching is performed by the spinning filterwheel 16 such that, for a moment in time, the filter wheel rotationexposes an optical filter (FIG. 4 item 52) to light (FIG. 1 item 10) fora gas of interest, then for a roughly equal interval of time, the filterwheel exposes a reference filter (FIG. 4 item 54) to the same light(FIG. 1 item 10).

The GFCr methodology is provided in this embodiment as well. Expandingon the DOAS example above, a CO₂ filter 53 can be paired with anothersimilar characteristic CO₂ filter 52 with the difference that the CO₂filter 52 has a windowed small cell 70 that contains a sample of CO₂gas. The amount of gas in the cell 70 is chosen based on the amount ofoptical depth that is desired with which the non-celled optical path iscompared. The CO₂ filter 53 must have balancing windows 78 of the sameoptical characteristics as the gas cell 70 in order to make the amountof light between both light paths roughly equivalent. An alternativeembodiment to the balancing windows 78 can use a second gas cell 70 inplace of the balancing windows 78, but with all air evacuated to avacuum, or air replaced with nitrogen or other inert gas at partialpressure to provide the optical balance. If a gas is used to fill thebalancing cell, the gas cannot have absorption characteristics similarto the gas of interest being measured.

The balancing windows 78 are added to create an optical balance for thetwo CO₂ detection paths in the example given, such that the onlydifference in intensity of light to the detector 50 between the twopaths is a change in concentration of the gas of interest. For a periodof time, the light travels through the CO₂ filter 52 with CO₂ gas cell70 and reaches the detector 50. In another time interval ofapproximately same length, the light will travel through the other CO₂filter 53 with balancing windows 78 and on to the detector 50. Since thegas cell 70 contains a known concentration and corresponding opticaldepth of a sample of CO₂, the amount of light in the filter 52 to gascell 70 to detector 50 path of light exists as a reference to which theamount of light from light path filter 53 to balancing windows 78 todetector 50 is compared. The amount of absorbance from each CO₂ lightpath is compared to determine a concentration of CO₂ in this example. Aswith the DOAS method of detection, light path switching is accomplishedby the spinning reflector 62 that provides light to each mentioned pathfor a period of time in addition to making light paths for other gassampling paths of this embodiment.

The unique advantage of GFCr is that any interferences to measuring aconcentration of CO₂ in this example appear in both CO₂ light paths andtherefore is commonly rejected among both light paths. Common moderejection of interferences does not necessarily happen with the DOASmethod of detection of gases, because of the use of a reference filterat a different wavelength, an interference could conceivably absorblight at the reference wavelength but not at the wavelengthcorresponding to the gas of interest. Also, the characteristics of thereference filter 54 are different from the other filters 52,53, andcreate a situation where different filters 52,53,54 pass differentwavelengths of light, to which the detector 50 will have greater orlesser sensitivity to such wavelengths. With proper optimizations, theseeffects may be minimized, but not eliminated.

It should be noted that it is not necessary to have both DOAS and GFCrmethodologies utilized in an embodiment in order to obtain reasonablemeasurements of concentrations of gases of interest. However it isdesirable to have both when economically feasible in order to providefor improved precision and accuracy of measurements. Furthermore,although an example was given here for CO₂, it is possible to utilizeGFCr for other gases including but not limited to carbon monoxide (CO),methane (CH₄), and any gas of interest that can be stored over longperiods of time in a gas cell without the reference gas of interestdegrading, attacking the walls of the cell and compromising the sample,or the reference gas combining with contaminants within the cell causingthe reference concentration to no longer be known. GFCr methodology alsois beneficial for speciation of hydrocarbons, as the gas cell 70 can beutilized as a sort of notch filter to indicate a particular gas ofinterest from a group of gases such as hydrocarbons.

Referring back to FIG. 6 the processor 92 of the embodiment, coupledwith the appropriate instruction set contained within memory 104, can becapable of conducting either DOAS, GFCr, or simultaneously bothmethodologies of detection of concentrations of gases and then applyingthe concentrations to a combustion equation. Previous art in this fieldof invention has documented combustion equations that utilize ratioingconcentrations of gases of interest relative to carbon dioxide (CO₂) tocorrect for any dilution effects in the exhaust stream of the vehiclebeing tested. The memory 104 can contain combustion equations unique todifferent fuels used to power vehicles that are tested by this preferredembodiment. Determination of the type of fuel used to power a testedvehicle can be done in the processor 92 at the time of measurement ofthe tailpipe emissions, or after emissions testing activities haveconcluded at the monitoring site in a centralized data processingfacility. A method for determining the type of fuel of a vehicle isdisclosed in U.S. patent application Ser. No. 09/928,720 entitled“METHOD AND SYSTEM FOR DETERMINING THE TYPE OF FUEL USED TO POWER AVEHICLE”, filed Aug. 13, 2001, the disclosure of which is herebyincorporated by reference in its entirety.

FIG. 9 illustrates a preferred embodiment including a light source 120capable of emitting at least one beam of light 122 having known emissionintensities corresponding to one or more of infrared, visible, andultraviolet spectra. The system also includes a reflection unit 124, adetection unit 90 capable of receiving the beam and measuring receivedintensities corresponding to the light spectra, and a processor 92capable of comparing received intensities and identifying aconcentration of a gas of interest. The light 122 is transmitted througha gas, such as air containing vehicle emissions, reflected, thendetected for analysis and measurement of the amount of absorption thathas occurred at known wavelengths. The amount of absorption may be usedto determine concentrations of gases corresponding to the specificwavelengths.

Preferably, as illustrated in FIG. 10, the system also includes a firstreflector 130 positioned to receive the beam 128 from the light source120 and reflect the beam 132 toward the reflection unit 124. Thereflection unit 124 is positioned to receive the beam 132 from the firstreflector 130 and reflect the beam 134 toward a second reflector 136.Also preferably, the second reflector 136 is positioned to receive thebeam 134 reflected by the refection unit 124 and reflect the beam 138toward the detection unit 90. In a preferred embodiment, each reflector130,136 comprises an off-axis paraboloidal mirror, however a sphericalor other similar mirror could be used.

Referring to FIG. 11, a paraboloidal mirror 180 has the property thatlight rays 182 emitted from and diverging from a small spot of a lightsource 184 placed near the paraboloidal mirror 180 focus 186 arereflected into a beam of rays 188 nearly parallel to the axis ofrotation 190 of the mirror.

Conversely, as illustrated in FIG. 12, a beam of light rays 192traveling nearly parallel to the axis of rotation 190 of a paraboloidalmirror 180 become rays 194 reflected toward and concentrated into asmall spot near the paraboloidal mirror focus 186. The significance of alight beam of nearly parallel rays 192 is that the intensity of thelight beam changes very little over a great distance, a desirable traitfor long path, open-path gas detection systems. Off-axis paraboloidalmirrors have the advantage that the light source or detection unit maybe located to the side of the reflected beam instead of in its midst.This means that the full diameter of the mirror can be used for theoptical measurements. Layout of the source and detector components isalso simplified. Spherical mirrors are more “fuzzy” at the focus, andincoming/outgoing light rays are not nearly as parallel as with theparallel rays 192 of the paraboloidal mirror 180. Light rays that do nottravel in the parallel path are lost from the optical path and as aconsequence, are part of the reduced efficiency of an optical systemthat utilizes spherical mirrors. Nonetheless, other factors such asavailability of product, production cost, etc. all factor in thedecision whether to use the preferred paraboloidal mirror 180 forsending/receiving light in the embodiment, or utilize spherical mirrorsin their place.

Returning to FIG. 10, a beam of light travels along an optical path 128,132, 134, and 138 from the light source 120, to the first reflector 130,to the reflection unit 124, to the second reflector 136, to thedetection unit 90. In this embodiment, the system also includes, as seenin FIG. 13, one or more additional light sources 144, 146, each capableof emitting a beam of light 148, 152 having known emission intensitiescorresponding to one or more of infrared, visible, and ultravioletspectra, as well as one or more beam splitter/combiners 140, 142, ifnecessary, positioned to direct beams 148, 152 from the additional lightsources 144, 146 along essentially the same optical path 154, 132, 134and 138 as illustrated in FIG. 10. A beam of light fromsplitter/combiner 140 can follow optical path 150 to splitter/combiner142. The beam splitter/combiners 140, 142 may be neutral densityfilters, or alternatively they may be wavelength sensitive beamsplitter/combiners, such as dichroic beam splitter/combiners.

In another embodiment, illustrated by FIG. 14, the light sources 10, 12,beam splitter/combiners 140, 160, infrared detecor 50, and spectrometer43 are positioned so that ultravoilet light beam 212 from source 12 istraveling along essentially the same optical path, but in the oppositedirection from infrared light beam 14 from source 10. This innovation isreferred to herein as “opposed sources”. An embodiment using opposedsources may eliminate the need for additional expensive, lightattenuating components. For instance, of ultravoilet light 212 isdirected towards, instead of away from, the infrared detector 50, thesignal from the infrared detector 50 can degrade. If light 212 from anultraviolet source 12 is traveling in the opposite direction (opticalpath 132, 134) from the light 14 emanating from the infrared source 10(optical path 216, 214), the ultraviolet light 212 is naturally keptaway from the infrared detector 50 without the use of additionalwavelength dependent filters or beam splitter/combiners. Light sources12, 10 and detectors 43, 50 need to be matched with optical componentsof corresponding F-numbers for efficient light transmission. Anembodiment using opposed sources, and first and second reflectors 130,136 of significantly different F-number, allows the sources or detectorsrequiring a higher F-number to be matched with the reflector with thehigher F-number, and the sources and detecors requiring a lower F-numberto be matched with the reflector with the lower F-number. Thiseliminates the need for additional optical components for F-numbermatching. Finally, opposed sources may significantly simplify componentlayout and reduction of thermal and electrical interference amongcomponents.

FIG. 13 shows one possible arrangement of three sources 120, 144 and146. In one preferred configuration, the source 120 is an infraredsource, the source 144 is a visible light source, and source 146 is anultraviolet light source. In this example, ultraviolet light reflectsoff splitter/combiner 142 but does not pass through anysplitter/combiners. The infrared light passes through twosplitter/combiners. However, the arrangement of these sources may beinterchanged in any combination, and one or more source types may beomitted entirely.

FIG. 14 depicts an ultraviolet source 12 and an infrared source 10. Theultraviolet source 12 could also be combined with a visible light sourcein a manner similar to the combination shown in FIG. 1, either using apass through ultraviolet source or by providing an additionalsplitter/combiner to combine the ultraviolet and visible light.

Thus, the many features and advantages of the invention are apparentfrom the detailed specification, and thus, it is intended by theappended claims to cover all such features and advantages of theinvention which fall within the true spirit and scope of the invention.Further, since numerous modifications and variations will readily occurto those skilled in the art, it is not desired to limit the invention tothe exact construction and operation illustrated and described, andaccordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

What is claimed is:
 1. An optical system for a gas component analysis, comprising: a first emitter located on a first side of a vehicle path for emitting a first light beam having a first spectrum across the vehicle path; a first receiver for receiving the first light beam; a spinning filter wheel that filters the beam from the first emitter before the beam crosses the vehicle path; a second emitter located on the first side of the vehicle path for emitting a second light beam at a second spectrum across the vehicle path; a second receiver for receiving the second light beam; a third emitter for emitting a third light beam; a third light receiver for detecting the third light beam, wherein the third light beam travels along a third path, and at least a portion of the third path overlaps with at least a portion of the second path; and a reflector that is a retroreflective assembly having at least three reflective faces, and wherein at least one of the beams travels across the road at a first height above the road, and returns across the road at a second height above the road different from the first height.
 2. The system according to claim 1, wherein the light beam is projected across a vehicle path, and the first and second emitters and first and second receivers are located on one side of the vehicle path, and wherein the system comprises a reflector located at the other side of the vehicle path to direct the first and second beams from the first and second emitters to the first and second receivers respectively.
 3. The system according to claim 1, wherein the second emitter is one of an infrared, ultraviolet, or visible light emitter.
 4. The system according to claim 1, wherein at least a portion of the first beam overlaps at least a portion of the second beam.
 5. The system according to claim 1, wherein the first emitter is an infrared emitter, and wherein the second emitter is an ultraviolet light emitter, and wherein the third emitter is a visible light emitter.
 6. The system according to claim 5, wherein at least a portion of the third beam overlaps at least a portion of at least one of the first and second beams.
 7. The system according to claim 1, wherein the wheel is located proximate to the first emitter.
 8. The system according to claim 7, wherein the emitting means emits infrared light.
 9. An optical system for a gas component analysis, comprising: a first emitter located on a first side of a vehicle path for emitting a first light beam having a first spectrum across the vehicle path; a first receiver for receiving the first light beam; and a spinning filter wheel that filters the beam from the first emitter before the beam crosses the vehicle path; wherein the spinning filter wheel has a plurality of filter elements, and wherein the filter elements are quarter circular in shape.
 10. The system according to claim 9, wherein the filter elements are removable.
 11. The system according to claim 9, wherein the number of filter elements is at least four.
 12. The system according to claim 9, wherein the filter elements are disposed at regular angular intervals around the wheel.
 13. The system according to claim 9, wherein at least one set of the filter elements are disposed in pairs so that both elements of the pair filter the first light beam simultaneously.
 14. An optical system for a gas component analysis, comprising: a first emitter located on a first side of a vehicle path for emitting a first light beam having a first spectrum across the vehicle path; a first receiver for receiving the first light beam; a spinning filter wheel that filters the beam from the first emitter before the beam crosses the vehicle path; and a synchronization feature on the wheel and a controller that interacts with the synchronization feature to measure the speed of rotation of the wheel.
 15. The system according to claim 14, wherein the first emitter is an infrared emitter.
 16. The system according to claim 14, further comprising a synchronization feature on the wheel that determines the position of the wheel indicating a filter position.
 17. The system according to claim 14, wherein the synchronization feature is a hole that passes through at least a portion of the wheel. 